This Phase II study will provide for a clinical evaluation of the use of a portable fractional carbon monoxide uptake device to predict and diagnose PCP in high risk AIDS patients. The clinical trial will evaluate the efficacy and technical performance of using a novel FU(CO) measurement that is highly reproducible and conducive to employing a small, portable pulmonary function device in three different approaches: 1) comparison of the FU(CO) measurement of a symptomatic patient with matched normal values, 2) comparison of the FU(CO) measurement of a symptomatic patient with the patients' own historical baseline, and 3) periodic monitoring of a patients' FU(CO) to predict onset of symptomatic disease. These studies will include separate analysis of male and female patients since these groups may present different diagnostic challenges. This is especially important because of the alarming rate of increase in HIV disease among women.